1. Field of the Invention
The present invention relates to a reciprocating screw (RS) injection unit and is particularly, but not exclusively, applicable to a method of operating such an RS injection unit in a plastic injection molding machine.
2. Related Art
In a conventional RS unit, a feedscrew is rotated intermittently, thereby limiting the amount of time available for plasticizing the next injection shot. The waiting time between driving operations is usually called the recovery time.
During the injection phase of operation of a conventional RS injection unit, the non-rotating screw is advanced to inject the melt into the mold. As the flights of the screw pass under the open hopper feed throat, unmelted resin granules fall in between the flights, becoming available for plasticating when the screw resumes rotation. However, in some applications, such as those with high speed injection rates, the high speed translation of the screw relative to the hopper feed throat does not allow for a complete filling of the spaces defined between the flights. Therefore only partial filling of the RS screw is achieved.
FIG. 4 shows a sequence of operation chart for a typical conventional (non-continuous) recovery cycle for an RS type injection unit. The process begins when a shut off valve opens (“S/O” means “shut off” and refers to the valve in the nozzle that controls the resin flow into the mold). Melt is then injected into the mold. The pressure on the melt in the mold is maintained for a period, after which pre-pullback begins. Upon completion of pre-pullback the shut off valve closes. As screw recovery begins, the screw speed sharply increases and plateaus during screw recovery. The screw speed subsequently decreases and stops whereafter post-pullback begins. As the injection unit goes through these cycle steps, the clamp unit holds the mold closed during injection, hold and cooling of the part, and then opens the mold to eject the part. After this the mold is closed again, ready to repeat the cycle.
Injection molding RS units are well known in the art. For example, U.S. Pat. No. 2,629,132 to Willcox discloses an early RS injection unit, wherein a motor turns the screw to cause plastic pellets beneath the hopper to be conveyed towards a bore where melted plastic accumulates. The screw is then translated by a piston to push the melted plastic from the bore into the mold via the nozzle. The feedscrew's shank (to the right of the flights) seals in the bore, thereby blocking off the hopper inlet. The feedscrew is then retracted by the piston and is rotated during the retraction by the motor to cause plasticizing.
Two practical deficiencies arise from this disclosure. First, during the translation of the screw to inject the material into the mold, the leading end of the screw makes no seal with the inside surface of the bore until it has reached the end of its stroke when it seals with a bushing. Therefore, some of the melted plastic in the bore is able to flow backwards across the flights of the screw as it moves to the left during injection. This loss of plastic makes controlling the shot size unpredictable from cycle to cycle. Second, as the screw begins to be retracted and rotated, the hopper inlet continues to be blocked by the screw shank, so no new material can enter the screw flights. Indeed, the hopper inlet does not become unsealed until the screw is fully retracted (to the right). In fact, the retraction of the screw with the hopper inlet sealed will cause air to be drawn into the bore via the nozzle and gate opening (in the mold) after the most recent molded part has been removed. This ingested air must then be displaced by the next shot of plastic as it is prepared in the bore by the feeding action of the screw when rotating in its fully retracted position. This may result in the formation of bubbles of air in the next molded part unless this air is vented prior to the next injection stroke of the screw.
U.S. Pat. No. 2,734,226 to Willert discloses an RS injection unit that includes a shut off nozzle that allows melt to be accumulated between the screw tip and the barrel head, and a back pressure generated by restricting the venting of the oil from the injection pressure as the screw is pushed rearwards by the accumulating melted plastic in front of it. The patent also discloses continuous rotation of feed screw. Again, the leading end of the screw makes no seal with inside surface of the barrel bore, thereby allowing some melted resin to flow backwards as the screw is translated during its injection stroke. Not only is shot control affected, but also only moderate injection pressures can be achieved due to the absence of a sealing feature. Furthermore, because the screw does not pause at the end of recovery (plasticating), the injection stroke of the screw starts immediately, and control of the shot size is poor.
U.S. Pat. No. 2,885,734 to Wucher discloses an injection unit that has a non-return valve disc attached to a diffuser head or screw tip that acts as a valve during the injection stroke, thereby inhibiting back flow of the material. This is an early example of a screw mounted non-return valve.
U.S. Pat. No. 3,002,229 to Friedrich discloses a die casting machine. A spring loaded shut off nozzle contains the melted material prior to injection. The screw does not translate but instead accumulates the melted material along the screw flights and in the space ahead of the screw where it is compressed while the shut off nozzle is closed. The patent discloses turning the screw continuously at two different speeds. A slow first speed is used for feeding and melting the material, and a higher second speed is used for injecting the material into the mold when the nozzle shut off is opened. Because the screw does not translate only a small shot size is prepared, and a relatively moderate injection pressure is generated since a non-return valve on the screw is absent.
U.S. Pat. No. 3,020,591 to Breher discloses an injection molding machine having a screw having a non-return valve and an accumulating chamber downstream of the screw. The patent also teaches that the feeder worm (screw) works uninterruptedly as it melts the plastic and feeds into the accumulating chamber. Injection of the melt is done by activating cylinders to cause the barrel head assembly to act like a piston inside the accumulating cylinder and displace its contents via the nozzle into the mold. A ball check non-return valve prevents backflow into the extruder cylinder.
U.S. Pat. No. 4,722,679 to Farrell discloses an injection unit that includes an RS extruder supplemented by two piston cylinder arrangements, one for accumulating the melt and filling the mold and the second for providing packing and suck back operations. The screw is fitted with a check ring (non return valve). The design provides a means to increase the shot size capacity of the unit without having to increase the size of the screw. The patent discloses that the screw recovery operation can begin as soon as the mold filling operation has been completed.
U.S. Pat. No. 5,112,213 to Oas discloses an RS injection unit with a check ring style non-return valve designed so that by rotating the screw briefly in the opposite direction prior to injection causes the check ring to seal against the tapered seat thereby reducing the possibility of leakage.
The Spirex website provides an example of a check ring style non-return valve .(F-LOC) that features an interlocking ring which turns with the retainer and plasticating screw as it rotates. Because the ring is not rotating against the front seat of the retainer, adhesive wear problems in this area are eliminated.
The Plastics Technology online article “Know-How: Injection Molded Furniture in a Slump” explains the intrusion process, sometimes called flow molding, whereby an injection unit of limited plasticizing capacity can be used for molding parts requiring larger shot sizes than the unit is able to supply conventionally. The article discloses a known process in which the mold is partly filled by turning the screw to pump plastic under low pressure into the mold. Specifically, the screw rotates without moving forward with the screw used as an extruder prior to its use in injection as a ram. The injection function carries out the final filling and packing steps.